1. Field of the Invention
The present invention concerns a high power semiconductor laser used in the optical communications, and more particularly a method of fabricating a high power semiconductor laser with the ridge type waveguide which is provided with ion implanted regions formed symmetrically at both sides of the ridge with the same interval.
2. Technical Background
The semiconductor laser lasing at wavelength of 0.98 .mu.m (hereinafter referred to as 0.98 .mu.m semiconductor laser) is used as a pumping light source for erbium (Er) doped fiber amplifiers. Such semiconductor laser is mostly built by ridge waveguide (RWG) structure, since it has a low optical power density at the output facet compared to other structures so that the energy level of the catastrophic optical damage (COD) becomes high. However, the semiconductor laser of RWG structure is based on weaking index-guiding, so that the fundamental lateral mode may not be maintained, but mixed with a higher-order lateral mode, when it is operated at high output power. This higher-order lateral mode emerge results in the kinks and beam steering of the optical output without being linearly proportional to the drive current, thereby degrading the usability of the semiconductor laser as the pumping light source. Especially, it irregularly varies the optical coupling efficiency between the semiconductor laser and the optical fiber. In order to prevent such higher-order lateral mode lasing, there have been proposed ion implanted regions formed in the channel regions of the semiconductor laser, so that it may make the fundamental lateral mode lasing regardless of the drive current and the optical output power.
Describing the conventional method of fabricating the 0.98 .mu.m of RWG structure in connection with FIGS. 1A to 1E, sequentially deposited by crystal growing over a compound semiconductor substrate 100 are a lower cladding layer 101, an active layer 102, a first upper cladding layer 103, an etching stop layer 104, a second upper cladding layer 105 and an ohmic contact layer 106, as shown in FIG. 1A. The active layer may be made of a single layer or a composite layer consisting of a lower optical waveguide layer, an quantum wll (QW) active layer and an upper optical waveguide layer.
Referring to FIG. 1B, the ohmic contact layer 106 is covered by an insulating layer 107, and then photo lithographically masked by a photoresist pattern (not shown) to define the channels 108 having widths of about 20 .mu.m and the ridge 109 having width of 2 to 5 .mu.m along the cavity of the laser. The insulating layer 107 is etched through the photoresist pattern to expose the parts of the ohmic contact layer 106 over the channels 108. Then removing the photoresist pattern, the insulating layer 107 is used as the etching mask to subject the second upper cladding layer 105 to wet or dry etching to expose the etching stop layer 104.
Subsequently, another thick photoresist pattern 110 is deposited over the ridge 109 and parts of the first upper cladding layer 103 at both sides of the ridge 109 by means of photolithography, as shown in FIG. 1C. In this case, the width "A" of the photoresist pattern 110 covering the parts of the first upper cladding layer should be the same at both sides of the ridge 109 in order to have the ion implanted regions formed symmetrically about the ridge 109.
Si, B or H ions are implanted into the parts of the first upper cladding layer 103 and active layer 102 below the channels so as to form the ion implanted regions 111, as shown in FIG. 1D. The depth of the ion implanted region 111 depends on the acceleration energy of implanting ions, which is adjusted considering the thickness of the first upper cladding layer to make the ion implanted region penetrate the active region up to the upper part of the lower cladding layer 101.
Referring to FIG. 1E, after forming the ion implanted regions 111, the upper surface of the semiconductor laser is covered by an insulating layer 112 formed of SiO.sub.2 or Si.sub.3 N.sub.4 except for the parts of the upper surface of the ridge serving as the current paths. Then, through the lift-off and gold plating processes are formed p.sup.- electrode 113 over the whole surface of the exposed parts of the ridge and n-electrode 114 over the underside of the substrate.
The ion implanted region 111 serves to absorb lights of 0.98 .mu.m wavelength. In the semiconductor laser of RWG structure, the optical field distribution of the higher-order lateral mode is spread towards the channels, so that the lights of the first-order higher lateral mode are continuously absorbed, resulting in optical energy loss over the threshold value and thus cutting off the first-order higher lateral mode. Namely, it becomes possible to generate only the fundamental lateral mode lasing, which undergoes relatively less energy loss by the ion implanted region having the selective light absorption for the lateral modes.
When forming the ion implanted regions to absorb the first-order higher lateral mode lasing, the width of the ridge between the channels of the RWG semiconductor laser is 2 to 5 .mu.m, and the diameter of the laser beam of the fundamental lateral mode about 5 .mu.m regardless of the ridge width. Hence, the part of the ion implantation mask to cover the ridge should have the width greater than the ridge width to achieve the desired ion implantation. Besides, it is hardly possible due to the mask alignment error to make the photoresist pattern 110 cover the same width "A" at both sides of the ridge 109 in order to obtain the symmetrical ion implanted regions about the ridge. If the ion implanted regions are not symmetrical about the ridge, the fundamental lateral mode is absorbed by the ion implanted region formed nearer to the ridge under the high injection current, reducing the electro-optical conversion efficiency, and thus degrading the laser characteristics.